1. Field of the Invention
The present invention relates to a process for forming a highly absorbent starch and the product formed by said process.
2. Description of the Related Art
Formation of carboxymethyl starch is known to those skilled in the art. As a general rule, carboxymethyl starch is suitably prepared by the reaction of starch with an alkali metal hydroxide such as sodium hydroxide whereby alkali starch is formed. The alkali starch is then reacted with a chloroalkanoic acid such as monochloroacetic acid to form the carboxymethyl starch. As normally produced, the carboxymethyl starch is water-soluble. Accordingly, the carboxymethyl starch is generally produced and recovered from a non-aqueous media such as an alcohol. The carboxymethyl starch so produced is known for use as a water-soluble thickening agent.
Unfortunately, the degree of thickening achievable through the use of the carboxymethyl starch described above is limited. For numerous applications it is desirable to employ a thickening agent with greater thickening powers than can be achieved with conventional carboxymethyl starch. Accordingly, U.S. Pat. No. 3,345,358 issued Oct. 3, 1967 to Inklaar describes a method of preparing a gel-forming derivative of polysaccharides such as carboxymethyl starch. The method involves acidifying finely divided carboxymethyl ethers of polysaccharides by treating them with acid in methanol or other water-miscible organic liquid medium. In this manner acid carboxymethyl groups are formed on the material. The material is held under acidified, non-hydrolyzing conditions to bring about the formation of ester bonds whereby constituent macro molecules of the material become crosslinked one to another. The material is then neutralized with an alkali. The derivatives so produced are described as being capable of forming a gel upon addition to water. The gels are described as having a smooth creamy or salve-like consistency and as being easily spreadable. The gels are useful as bodying, gelling, emulsifying, suspending or thickening agents for a wide variety of aqueous liquids.
Great Britain Patent No. 1 550 614 published Aug. 15, 1979 describes an absorbent material suitably formed from starch. Described is the crosslinking of starch with a chloroalkyl oxirane such as epichlorohydrin. The crosslinked starch is then treated with sodium hydroxide and monochloroacetic acid and slurried in hydrochloric acid to recover the starch-based absorbent.
Both of the above references described methods for producing crosslinked starch derivatives. Unfortunately, both of the references describe methods which are generally cumbersome in that they involve multiple steps. Specifically, the first reference involves a separate acidifying step with the second reference describing the use of a crosslinking agent to crosslink the starch prior to forming a starch derivative. It is desirable to provide a direct method for forming a crosslinked starch derivative which method is less cumbersome than the methods previously described.
U.S. Pat. No. 3,379,720 issued Apr. 23, 1968 to Reid is directed to water-soluble polymers and the process of preparing such polymers. Reid describes a process of preparing water-soluble polymers which can be rendered water-insoluble by curing. The method describes acidifying ethers and esters of polysaccharides which contain carboxylate groups, such as cellulose, starch and natural gums, until the ratio range of the free acid form to the salt form of its carboxyl groups is about 0.07 to 1-3 to 1, removing the excess acid and drying. The resultant modified polysaccharide is described as being water-soluble and capable of being insolublized by curing. Again, Reid describes a method involving a separate acidification step.
U.S. Pat. No. 4,483,950, issued Nov. 20, 1984, to Fanta et al., describes using highly modified, low molecular weight starches as extenders for starch-based superabsorbents. Dextrinized starches are described as synergistically interacting with the superabsorbents to permit dilution without a commensurate reduction in the water absorbency.
U.S. Pat. Nos. 3,981,100, issued Sept. 21, 1976 and 3,935,099, issued Jan. 27, 1976, to Weaver et al. describe absorbent starch-containing polymeric composition which are prepared by graft polymerizing acrylonitrile onto starch followed by saponification of the starch-poly(acrylonitrile) graft polymer. The sponified polymer may then be recovered by drying.